1. Field of the Invention
This invention relates to compositions and processes for forming a protective coating on metal, particularly aluminum and/or aluminum alloy, surfaces. The coating formed is solid and adherent, often although not always colored, provides good corrosion protection to the metal surface, and functions as an excellent base for painting or similar organic based protective coatings. The coatings formed, when applied to substrates of aluminum and its alloys, usually impart as much corrosion protection as do conventional coatings formed by use of hexavalent chromium containing compositions, but the compositions and processes according to this invention cause less potential environmental dam-age because they do not need hexavalent chromium or other identified major pollutants.
1. Discussion of Related Art
The art believed to be most closely related is that described in WO94/00619, particularly Table II thereof This reference teaches that high quality coatings can be formed on metal substrates by contacting them with aqueous compositions believed to contain cobalt(III) complex anions formed by reaction among cobalt(II) salts, carboxylate ions, and various other ions in the presence of an oxidizing agent. Other closely related art includes U.S. Pat. No. 3,905,838 of Sep. 16, 1975 to Ito and U.S. Pat. No. 5,298,092 of Mar. 29, 1994 to Schriever.
Many of the compositions taught in this related art, although they have avoided the use of hexavalent chromium and other pollutants of the general environment, nevertheless can have adverse environmental impacts on the immediate working area for the process and any workers in this area. For example, many of the formulations previously used include high concentrations of ammonia, which causes at least a severe odor nuisance and possibly a serious health hazard to workers in the vicinity, unless expensive ventilation equipment is installed in the process area. Furthermore, the concentration of ammonia in aqueous solutions is difficult to maintain constant, as is desirable for achieving the most consistent results from the process, at the high levels previously recommended by some related art. In addition, some of the previously recommended compositions contain both nitrite ions and amines, which are generally believed to be readily capable of reacting to form nitrosamines, many of which are known carcinogens.